1. Field of the Invention
The present invention pertains to antennas and antenna circuits for transmission of RF (radio frequency) signals. More specifically, the invention pertains to a mobile antenna and antenna circuit for automatically resonantly tuning the antenna circuit at a specified frequency.
2. Description of the Prior Art
The strength of an electromagnetic field radiating from a section of wire carrying radio frequency current depends, among other things, on the amount of current flowing therein. Other things being equal, the field strength is directly proportional to the flowing current. Generally speaking, it is desirable to make the current flowing as large as possible. An RF antenna circuit contains an inductor and a capacitor in series. Resonance is a special case in a series circuit containing both inductance (L) and capacitance (C) in which its inductive reactance (X.sub.L) and its capacitive reactance (X.sub.C) are equal. It is well known that power radiated by an antenna operated at resonance is substantially increased over one not operated a resonance. Receptivity of a resonant antenna in receiving radio energy is also increased. Thus, resonance is desirable in an antenna circuit.
Broadcasting stations induce in the antenna circuit emf's of frequencies equal to the carrier frequencies of the stations. When the circuit is tuned so that the circuit is in resonance with the frequency of any desired station, the current corresponding to that particular frequency is large, and a large potential appears across its capacitor. But since at any one setting the circuit is in resonance for one frequency only, other stations will develop only negligibly small currents and potential differences and hence will not be heard.
Especially in mobile systems, it is often necessary to provide a single antenna, usually of fixed small physical length which is capable of satisfactory operation over a broad band of frequencies. The antenna operates in such a manner that its impedance match with a transmission line at each of the frequencies satisfactory for operability. This type of operation is most often used in aircraft, automobiles, trucks, small boats or other moving vehicles, where for reasons of economy of space and weight, a single antenna system must be used.
In many instances these antennas of fixed length have a maximum dimension which is small compared to the operating wave length and have impedance characteristics which vary rapidly with frequency. Good operation is normally only achieved over a relatively narrow range of frequencies, if fixed matching networks and tuning elements are used. For use over a range of frequencies, these antennas require retuning of the matching circuit of the antenna system as the frequency of operation is changed.
The advantages of being able to quickly and exactly resonate an antenna to any frequency over a certain range has long been recognized. This is particulary true of mobile operation which incurs constant change of environment resulting in continual changes in the resonant characteristics of the antenna and antenna circuit. Thus, many attempts have been made to provide an easily and quickly resonating system. U.S. Pat. Nos. 2,810,070 and 2,855,599 disclose circuits utilizing automatic antenna tuners designed for this purpose. In U.S. Pat. No. 3,381,222 the movement of a loading coil in an antenna to place the antenna in resonance with the RF signal is controlled by a logic circuit automatically responsive to signals from a resonance detector unit. The loading coil inductance is changed by moving a ferite slug into the center of the coil. However, ferite coil inductors are very inefficient at high power levels. U.S. Pat. No. 3,475,703 discloses an automatically controlled antenna tuning system which utilizes a phase discriminator or detector for developing a signal to control resonance. However, the control circuitry is elaborate and the power handling capability is limited through the use of a variable capacitor or variable inductor.